This invention relates to an electrolytic process for producing lead and tin sulfonates for use in solder plating to form coatings with smaller counts of radioactive .alpha. particles than heretofore; plating baths containing those lead and tin salts having a reduced content of radioactive isotope impurities such as uranium and thorium; and electrodeposits formed by solder plating whose radioactive .alpha. particle counts are less than 0.1 CPH/cm.sup.2.
A new aspect of the highly developed electronic industry today is the use of tinning or solder plating in precoating electronic components to enhance their solderability. Formerly borofluoride baths were used for solder plating. They have largely been supplanted by less toxic baths of organic sulfonates as an antipollution measure. Fluorine, one of the elements constituting borofluoric acid for the former baths, is highly toxic and involves difficulties in the wastewater disposal. Many reports have thus far been made on the plating techniques using those organic sulfonates and also about the additives for them.
The organic lead and tin sulfonates to be employed in solder plating solutions are usually prepared by heating and dissolving the oxide, hydroxide, or carbonate of such a metal in an organic sulfonic acid. The oxides, hydroxides, and carbonates of those metals contain much uranium (U) and thorium (Th), both of which are alpha-ray sources. Thus the greatest disadvantage of the ordinary chemical dissolving process stems from the contamination of the lead and tin sulfonates with the impurities; the electrodeposits formed by solder plating with those salts produce .alpha. rays abundantly enough to invite soft errors of memory devices.
We have already filed a patent application (Kokoku No. 4624/1991) for an electrolytic process for producing organic lead and tin sulfonates, etc. using anion-exchange membranes, with 99.99%-pure metallic lead and tin as anodes. Metallic lead and tin as such contain uranium and thorium, both .alpha.-ray sources. Therefore, although the patent process gives solder plating electrodeposits of somewhat smaller counts of radio-active .alpha. particles than the conventional chemical dissolving method, a further improvement in the process is required for greater reliability of memory devices.
In view of these, the present invention aims at providing an electrolytic process for producing organic lead and tin sulfonates with reduced counts of radioactive .alpha. particles through removal of the radioactive isotopes, such as uranium and thorium, inevitably contained as impurities in lead and tin that are chief components of the coatings formed by solder plating, in order to realize solder plating with fewer occurrences of semiconductor memory errors than heretofore.